For many years a significant proportion of commercial FCC catalysts used throughout the world have been made by in situ synthesis from precursor microspheres containing kaolin that had been calcined at different severities prior to formation into microspheres by spray drying. One form of the calcined kaolin clay is referred to as metakaolin. The other form, obtained by calcination at a higher temperature, is referred to as spinel (or, alternatively, as kaolin calcined through the characteristic kaolin exotherm or fully calcined kaolin). Generally, these in situ fluid cracking catalysts microspheres composed of zeolite Y and an alumina rich silica-alumina matrix derived from calcined clay. It is well known that properties of the matrix can have a profound impact on the properties and performance of a zeolitic cracking catalyst. This is particularly true for in situ cracking catalysts where the zeolite Y is grown directly on/in the microsphere and is intimately associated with the matrix material.
Certain catalysts are made by the in situ route with microspheres which initially (before calcination and crystallization) contain a hydrous kaolin clay to spinel weight ratio between 40:60 to 50:50; the microspheres are then calcined at a temperature below the exotherm to convert the hydrous clay component to metakaolin. These catalysts will be referred to herein as type A catalysts. Another type of catalyst is made by spray drying hydrous kaolin microspheres, whereby calcined microspheres contain only metakaolin; spinel is not present. These catalysts will be referred to herein as type B catalysts.
The catalytic properties of such catalysts are influenced by the starting microspheres from which they are made. Type B catalyst has lower coke and dry gas selectivities than type A catalyst, but it is difficult to reduce sodium to low levels during manufacture and, in the absence of rare earth cations, it is not as stable as Type A. Type B catalyst also does not have as good a bottoms upgrading capability as Type A catalyst. The latter is more and easier to process than Type B catalyst but has a higher dry gas and coke selectivity.
See U.S. Pat. No. 4,493,902 for typical procedures used to make catalysts such as Type A catalysts using raw uncalcined (i.e., hydrated) kaolin and spinel as spray dryer feed, followed by calcination of the spray dried microspheres to convert the raw kaolin component to metakaolin and subsequent crystallization by reacting the microspheres in a seeded sodium silicate solution. The production of Type B catalysts is similar and involves using only raw uncalcined kaolin as spray dryer feed, whereby calcination of the resulting microspheres results in microspheres in which essentially all of the calcined kaolin is present in metakaolin form.
U.S. Pat. No. 5,395,809 describes improved catalysts that are more stable and easier to process than the microspheres used to produce Type B catalysts, yet substantially retain the selectivity benefits of Type B catalysts, while having bottoms upgrading capability similar to Type A catalyst but with lower coke and dry gas selectivities.
Applicants of said patent found that the proportions of hydrous clay and fully calcined clay contained in the microsphere prior to in situ zeolite growth will significantly affect the properties and performance of the resulting catalyst. Furthermore, they found that the resulting properties and performance attributes such as coke yield, bottoms upgrading, metals resistance, zeolite stability, activity and ease of sodium removal did not vary linearly with the proportions of hydrous clay and fully calcined clay. As a result there was a certain range or window where all or most of the desirable properties and performance attributes were at or near optimal. Applicants found that the boundaries of this window were defined by the weight ratio of hydrous kaolin to spinel and were approximately 90:10 to 60:40.
The preferred method for making such catalysts involved initially preparing microspheres composed of combinations of hydrous clay and spinel such that the initial hydrous clay content, expressed as weight percent, was greater than the spinel content and the microspheres, at this point of the process, were essentially free from metakaolin. The microspheres also contained a silica binder, usually greater than 5 wt % of the spray dried particles. The silica binder was provided by the addition of an alkaline sodium silicate solution. The microspheres were calcined at a predetermined temperature to convert the hydrous clay to metakaolin without significantly altering the spinel level. In situ Y zeolite FCC catalysts were then made from these microspheres by subsequent crystallization in a seeded sodium silicate solution and ion exchanged to reduce sodium level. These catalysts (hereinafter Catalyst C) were more stable and as active as Type B catalyst. Furthermore, sodium could be removed more easily during manufacture than with Type B catalyst. Also, Catalyst C had low coke and dry gas selectivities, similar to those of Type B catalysts. The ease of sodium removal and high activity with concurrent low dry gas and coke yields made these modified microsphere catalysts excellent candidates for high octane catalysts, high isobutylene catalysts and improved (compared to Type B) bottoms upgrading catalysts.
In recent years the oil refining industry has shifted to processing a larger quantity of resid due to the changing product slate and price structure of crude oil. Since the early 1980's many refiners have been processing at least a portion of residual oil as a feedstock in their units and several now run a full residual oil cracking program. Processing resid can drastically alter yields of valuable products relative to a light feed in a negative direction.
Several factors are important to resid catalyst design. It is highly favorable if the catalyst can upgrade bottoms, minimize coke and gas formation, maximize catalyst stability, and minimize deleterious contaminant selectivity due to metal contaminants in resid feedstocks such as nickel and vanadium. While catalysts A, B & C are commercially valuable, none of these in situ catalysts possessed such a combination of properties when used to crack resid feedstocks.
Following the inception of catalytic cracking by Houdry in the early 1900's where an acid treated clay was used, the first revolution in the art of catalyst technology was the use of synthetic silica-alumina. The use of silica-alumina which had much more acidic Bronsted and Lewis acid sites increased the cracking activity and selectivity of the process over the clays. The second revolution came with the advent of zeolites and the discovery that they could be applied to cracking. The clear advantage of the zeolite was that the non-selective cracking to coke and gas was greatly reduced owing to the discrete pore structure of the crystalline zeolite and the shape selective chemistry which they provided. With the thrust in modern refining to limit the amount of coke and gas so as to maximize gasoline production the designed use of silica-alumina in cracking catalysts has decreased (see A. A. Avidan in: Fluid Catalytic Cracking: Science and Technology.Studies in Surface Science and Catalysis vol. 76. Magee, J. S. and Mitchell, M. M. Eds.; Elsevier, Amsterdam; 1993). The use of added aluminas has also found merit in helping to boost a catalyst's activity since pure aluminas also posses acidic sites. The relative activity of a catalyst is roughly proportional to the total quantity of acid sites present. Unfortunately alumina characteristically contains a large fraction of Lewis acid sites relative to Bronsted type sites. Lewis sites have been shown to be largely involved in the chemistry of hydride abstraction and coke formation (see Mizuno et al. in Bulletin of the Chemical Society of Japan vol 49, 1976, pg. 1788-1793).
Fluid cracking catalysts which contain silica- alumina or alumina matrices are termed catalysts with "active matrix". Catalysts of this type can be compared with those containing untreated clay or a large quantity of silica and which are termed "inactive matrix" catalysts. Work done by Otterstedt etal. (Applied Catalysis vol 38, 1988, pg. 143-155.) clearly shows the disadvantage of active matrices for coke and gas production sometimes producing twice as much as the inactive formulation.
Aluminas have long been used in hydrotreating and reforming catalyst technology (see P. Grange in Catalysis Reviews- Science and Engineering vol.21, 1980, p. 135.). Aluminas, and particularly transition aluminas, in addition to displaying acidic character also posses high surface areas typically on the order of several hundred meters squared per gram. They may be well suited for catalyst applications such as those mentioned where a metallic component is to be supported on the substrate surface (alumina in this case). The high surface area of the host material allows for a more uniform, dispersed arrangement of the metal. This leads to smaller metal crystallites and helps to minimize metal agglomeration. Metal agglomeration or sintering is a leading cause of loss of activity since the activity for metal catalyzed reactions is proportional to the exposed metal surface area. When the metal "balls up" metallic surface area is lost and so is activity. In relation to catalytic cracking, despite the apparent disadvantage in selectivity, the inclusion of aluminas or silica-alumina has been beneficial in certain circumstances. For instance when processing a hydrotreated/demetallated vacuum gas oil (hydrotreated VGO) the penalty in non-selective cracking is offset by the benefit of cracking or "upgrading" the larger feed molecules which are initially too large to fit within the rigorous confines of the zeolite pores. Once "precracked" on the alumina or silica-alumina surface, the smaller molecules may then be selectively cracked further to gasoline material over the zeolitic portion of the catalyst. While one would expect that this pre-cracking scenario might be advantageous for resid feeds they are unfortunately characterized for the most part as being heavily contaminated with metals such as nickel and vanadium and to a lesser extent, iron. When a metal such as nickel deposits on a high surface area alumina such as those found in typical FCC catalysts, it is dispersed and participates as highly active centers for the catalytic reactions which result in the formation of contaminant coke (contaminant coke refers to the coke produced discretely from reactions catalyzed by contaminant metals). This additional coke exceeds that which is acceptable by refiners.